Contrary to reports, the Kelihos botnet has not crawled out of the grave, Microsoft said last week. But the company acknowledged that a new botnet is being assembled using a variant of the original malware.

The reappearance of a Kelihos-like army of hijacked computers shows just how difficult it is to eradicate a botnet, security experts said today.

"It's not possible in most cases," said Roel Schouwenberg, a senior researcher with Moscow-based antivirus company Kaspersky Lab, when asked whether killing a botnet was feasible. "What you're going for is disruption more than anything."

Liam O Murchu, manager of operations at Symantec's security response team, agreed and said that there was only one way to insure a botnet's death.

"If you get to the people behind it [through arrests and convictions], that will be the most successful," said O Murchu. "But international borders and the lack of cross-country cooperation makes that a difficult road to go down."

Kelihos was taken offline last September when Microsoft, using a federal court order, led efforts to shut down domains used by the command-and-control (C&C), severing links between the compromised computers and their order-giving master. Microsoft identified the alleged botmaster as a Russian programmer, Andrey Sabelnikov, in an amended complaint last week.

Talk of a Kelihos resurrection was sparked last week by Kaspersky, which said it had found signs of new malware built on the Kelihos code. The implication was that Kelihos had returned from the dead and was again spamming users.

"Kaspersky has reported no loss of control of the [Kelihos] peer-to-peer operations and Microsoft researchers have confirmed this week that the original Kelihos C&C and backup infrastructure remains down, but it appears [a] new botnet infrastructure may be being built with the new variant of Kelihos malware," said Boscovich in a Jan. 3 blog.

Kaspersky confirmed that on Monday.

"The botnet we took down is still under control and infected machines are not receiving commands from the C&C centre, so they are not sending spam," Alex Gostev, chief security expert at Kaspersky, said in a statement. "But new samples which are monitored by us continue to get orders from spammers and send spam so far. It means that we are dealing with another botnet."

The appearance of that new botnet illustrates the difficulty researchers, software vendors and authorities have in exterminating a botnet, something that Boscovich, who cited several takedown successes, acknowledged.

"Taking down a single threat has never been Microsoft's ultimate goal in our fight against botnets," said Boscovich. "Rather, [we hope] to transform the fight against cybercrime by developing, testing and advancing impactful and disruptive strategies. This is a long-term effort."

New botnets based on old-and-offline predecessors are not unusual: As Boscovich noted, the original Kelihos was probably developed using code for Waledac, a botnet that Microsoft and others brought down two years ago.

"We don't see who is behind each botnet, what we see is an evolution," said O Murchu. "A botnet brought down in some way may disappear for some months, but then reappear. In many cases, it's unclear if it's the same group or they sold their code to others [to modify]."

The struggle to eliminate a botnet has analogies in the non-digital world, said Schouwenberg. "It's like a big drug arrest where hundreds of kilos of cocaine are seized," he said. "It's damaging to the criminals, but it doesn't put them out of business."

The ideal solution is to find, arrest and prosecute botnet makers and operators, both Schouwenberg and O Murchu said. But that's not easy.

"It's a frustrating task," said O Murchu. "Researchers often know who is behind [a botnet], but to get action taken can take an incredibly long time. That's incredibly frustrating."

Schouwenberg and O Murchu each cited as an example the claim last month that several Russian hackers were responsible for the Koobface botnet. The five men identified by security experts as the brains behind the botnet have yet to be arrested or charged.

But the experts believed that takedowns are worthwhile, even if those efforts aren't completely effective.

"If the fear of being caught isn't applicable, then the best thing we can do is hit the 'reset' button for the bad guys, and make them start over with a new botnet," said Schouwenberg.